Column base structure

ABSTRACT

In a column base structure provided with a column base portion in which a joint metal fitting arranged above a base concrete is fixed to an anchor bolt protruding upward from the base concrete, and a column member which is joined its lower end portion to an upper surface of the joint metal fitting, a design basis strength F 1 ′ of the joint metal fitting is smaller than a design basis strength F 2  of the column member, a bending bearing force of the column base portion is smaller than an index bending bearing force of the column member which is calculated by multiplying a bending bearing force of the column member by a value obtained by dividing the design basis strength F 1 ′ by the design basis strength F 2 , and the column base portion yields before the column member according to an increase of the load applied to the column base structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a column base structure provided with a column base portion in which a joint metal fitting arranged above a base concrete is fixed to a leading end portion of an anchor bolt protruding upward from an inside of the base concrete, and a column member which is joined its lower end portion to an upper surface of the joint metal fitting.

2. Description of the Conventional Art

FIGS. 4 to 6 are views which are referred for describing a conventional column base structure 2.

The conventional column base structure 2 is structured, as shown in FIG. 4, such as to be provided with a column base portion 5 which is provided on a base concrete 3, and a steel frame column 4 (a column member) which is joined its lower end portion to a joint metal fitting 6 of the column base portion 5. The column base portion 5 has the joint metal fitting 6, a mortar 8, an anchor bolt 10, nut members 12 and 14, a washer 16 and a fixing plate 18.

The column base portion 5 of the conventional column base structure 2 is structured such that the tabular joint metal fitting 6 having both front and back faces is provided above the base concrete 3 via the mortar 8. The joint metal fitting 6 of the column base portion 5 is made of a metal, and a lower end portion of the steel frame column 4 having a length in a vertical direction in the drawing is joined by welding to an upper surface 6 a (a front surface) thereof.

Here, in FIG. 4, as a matter of convenience for description, a weld portion W is not drawn in a surface in a near side of the drawing of the steel frame column 4, however, the weld portion W is formed in the surface in the near side and the surface in the back side of the drawing, in the same manner as two side surfaces which are adjacent to these surfaces. Same applied to the other drawings such as FIGS. 5 and 6 in this regard.

Further, an upper end portion of the anchor bolt 10 protruding upward from the inner side of the base concrete 3 is inserted to a bolt through hole 6 b in a peripheral edge portion of the joint metal fitting 6 and a through hole of the washer 16. A male thread portion formed in an upper end portion of the anchor bolt 10 is threadably fastened to female thread portions of two nut members 12, whereby the steel frame column 4 is provided in a rising manner on the base concrete 3 via the weld portion W, the joint metal fitting 6 of the column base portion 5 and the mortar 8 so as to be fixed (refer, for example, to patent document 1).

Further, the male thread portion formed in the lower end portion of the anchor bolt 10 is loosely inserted to the through hole passing through in a thickness direction of the fixing plate 18 and is threadably fastened to the female thread portion of the nut member 14 in the upper surface side and the lower surface side of the fixing plate 18, in the base concrete 3, whereby the fixing plate 18 is integrally fixed to the lower end portion of the anchor bolt 10 in the base concrete 3.

Further, as the other conventional column base structure, there has been a column base structure employing a joint metal fitting having a different shape from a simple tabular shape, the joint metal fitting being constructed by a bottom plate portion, and a support table portion which is higher upward in a center portion of an upper surface of the base plate portion than a peripheral edge portion thereof and is formed as into a trapezoidal shape, and being structured such that a lower end portion of a steel frame column is joined by welding to an upper surface of the support table portion, in place of the joint metal fitting 6 mentioned above (refer, for example, to patent document 2).

The other conventional column base structure according to the patent document 2 is structured such that the upper end portion of the anchor bolt protruding upward from the inner side of the base concrete is inserted to the bolt insertion hole passing through in the thickness direction of the peripheral edge portion of the bottom plate portion in the joint metal fitting, and the male thread portion formed in the anchor bolt is threadably fastened to the female thread portion of the nut member, whereby the steel frame column is provided in a rising manner on the base concrete via the weld portion W, the joint metal fitting of the column base portion and the mortar so as to be fixed.

In the meantime, a building construction provided with the conventional column base structure 2 (refer to FIG. 4) is structured such that one having a weaker bearing force of the steel frame column 4 and the column base portion 5 in the column base structure 2 yield in advance so as to plastically deform, in the case that a load generating a great bending moment M which intends to rotate the steel frame column 4 is applied in a clockwise direction around a center O of rotation of the center portion between a pair of weld portions W of the steel frame column 4 to the joint metal fitting 6 in both side surfaces of the steel frame column 4, as shown in FIG. 5.

In the building construction provided with the conventional column base structure 2 mentioned above, in order to make the column base portion 5 yield before the steel frame column 4 so as to plastically deform, it is necessary to make the bearing force against the bending moment M of the column base portion 5 larger than the bearing force of the steel frame column 4.

Here, a bearing force (including a bending bearing force Mp and an axial bearing force Np mentioned later) of the steel frame column 4 is defined by a design basis strength F2 of the steel frame column 4, and the greater the design basis strength F2 of the steel frame column 4 is, the greater the bearing force of the steel frame column 4 is.

On the contrary, a bearing force (including a bending bearing force Mu and an axial bearing force Nu mentioned later) of the column base portion 5 is not defined by a design basis strength F1 of the joint metal fitting 6, but is defined by a strength of the joint metal fitting 6, a strength of the anchor bolt 10, and a fixing performance of the joint metal fitting 6 to the base concrete 3 via the mortar 8, the anchor bolt 10, the nut members 12 and 14, the washer 16 and the fixing plate 18.

Further, in the case that the conventional column base structure 2 is sectioned into two structures constructed by a column base structure 2A in which the design basis strength F1 of the joint metal fitting 6 is equal to or more than the design basis strength F2 of the steel frame column 4, and a column base structure 2B in which the design basis strength F1 of the joint metal fitting 6 is smaller than the design basis strength F2 of the steel frame column 4, the column base structure 2A has been widely employed conventionally.

The reason why the column base structure 2A has been widely employed conventionally is as follows. In the case that a load generating a great bending moment M which intends to rotate the steel frame column 4 is applied to the steel frame column 4 in the conventional column base structure 2B mentioned above, in a clockwise direction around a center O of rotation of the center portion between a pair of weld portions W of the steel frame column 4 to the joint metal fitting 6, as shown in FIG. 6, the weld portion W joining the steel frame column 4 to the joint metal fitting 6 of the column base portion 5 locally yields, so that there is fear that the weld portion W is broken and the steel frame column 4 and the joint metal fitting 6 of the column base portion 5 are separated.

On the other hand, in the case that the load generating the great bending moment M which intends to rotate the steel frame column 4 is applied to the steel frame column 4 in the conventional column base structure 2A mentioned above, in a clockwise direction around a center O of rotation of the center portion between a pair of weld portions W of the steel frame column 4 to the joint metal fitting 6, as shown in FIG. 5, the steel frame column 4 is bent in the clockwise direction at a portion above the weld portion W in place of the yielding of the weld portion W joining the steel frame column 4 to the joint metal fitting 6, so that there is not the fear that the weld portion W is broken and the steel frame column 4 and the joint metal fitting 6 of the column base portion 5 are separated.

As a result, in the conventional column base structure 2, the design basis strength F1 of the joint metal fitting 6 is equal to or more than the design basis strength F2 of the steel frame column 4.

Further, in the building construction provided with the conventional column base structure 2, a structural calculation thereof is carried out by a structural calculation device using a structural calculation program.

Here, the structural calculation program continuously carries out a calculation defined by Building Standard Law such as various bearing force determinations, a calculation of allowable stress and a calculation of critical bearing force, and admission decision thereof, on the basis of input data such as a scale and a shape of the building construction, a material and a dimension of the column base structure in the building construction and a beam and a column which construct an upper structure above the column base structure, and various load data, for example, a fixed load and a live load, and displays and/or prints all the results thereof in the structural calculation device.

Further, in the conventional structural calculation device, the values of the bending bearing force Mp and the axial bearing force Np of the steel frame column 4, the bending bearing force Mu and the axial bearing force Nu of the column base portion 5 are calculated by the conventional structural calculation program on the basis of the input data, the smaller one of the bending bearing force Mp of the steel frame column 4 and the bending bearing force Mu of the column base portion 5 is employed as a final bending bearing force Ms of the column base structure 2, and the structural calculation of the building construction is carried out by employing the final bending bearing force Ms as a reference of the admission decision.

PRIOR ART DOCUMENT Patent Document Patent Document 1: Japanese Unexamined Patent Publication No. 2003-232078 Patent Document 2: Japanese Unexamined Patent Publication No. 2003-336266 SUMMARY OF THE INVENTION Problem to be Solved by the Invention

In recent years, a high strength of the steel frame column 4 is going to be progressed, and a high strength material having a greater value of the design basis strength F2 is going to be employed as the material of the steel frame column 4. As a result, in order to make the design basis strength F1 of the joint metal fitting 6 larger than the design basis strength F2 of the steel frame column 4, it is necessary to employ the high strength material having the greater value of the design basis strength F1 as the material of the joint metal fitting 6. Therefore, there is a problem that a manufacturing cost of the joint metal fitting 6 becomes higher, and a manufacturing cost of the conventional column base structure 2 employing the joint metal fitting 6 also becomes higher.

Accordingly, the present invention is made by taking the problem mentioned above into consideration, and an object of the present invention is to provide a column base structure which can prevent a joint portion between a joint metal fitting and a column member from locally yielding and prevent the joint portion from being broken, in the case that a load generating a bending moment is applied due to an earthquake, and can prevent a manufacturing cost of the joint metal fitting and the column base structure from becoming higher.

Means for Solving the Problem

In order to achieve the object mentioned above, a column base structure according to the present invention is a column base structure comprising:

a column base portion in which a joint metal fitting arranged above a base concrete is fixed to an anchor bolt protruding upward from the base concrete; and

a column member which is joined its lower end portion to an upper surface of the joint metal fitting,

wherein a design basis strength of the joint metal fitting is smaller than a design basis strength of the column member,

wherein a bending bearing force of the column base portion is smaller than an index bending bearing force of the column member which is calculated by multiplying a bending bearing force of the column member by a value obtained by dividing the design basis strength of the joint metal fitting by the design basis strength of the column member, and

wherein the column base portion yields before the column member according to an increase of the load applied to the column base structure.

Further, in the column base structure according to the present invention, the joint metal fitting is formed into a plate shape which has both front and back surfaces and a uniform thickness.

Further, in the column base structure according to the present invention, the joint metal fitting has a bottom plate portion which has both front and back surfaces and a uniform thickness, and is formed into a plate shape, and a support table portion which has a height toward an upward direction in an inner side of a peripheral edge portion of an upper surface of the bottom plate portion, and a lower end portion of the column member is joined to an upper surface of the support table portion.

Effect of the Invention

According to the column base structure of the present invention mentioned above, in the column base structure provided with the column base portion in which the joint metal fitting arranged above the base concrete is fixed to the anchor bolt protruding upward from the base concrete, and the column member which is joined its lower end portion to the upper surface of the joint metal fitting, the design basis strength of the joint metal fitting is smaller than the design basis strength of the column member, the bending bearing force of the column base portion is smaller than the index bending bearing force of the column member which is calculated by multiplying the bending bearing force of the column member by the value obtained by dividing the design basis strength of the joint metal fitting by the design basis strength of the column member, and the column base portion yields before the column member according to the increase of the load applied to the column base structure. Therefore, it is possible to prevent the joint portion between the joint metal fitting and the column member from locally yielding and prevent the joint portion from being broken, in the case that the load generating the bending moment is applied due to the earthquake, and it is possible to prevent the manufacturing cost of the joint metal fitting and the column base structure from becoming higher.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a partly cross sectional side elevational view showing a column base structure 40 according to a first embodiment of the present invention;

FIG. 2 is a conceptual side elevational view for explaining a state in which a load generating a bending moment M is applied in the column base structure 40 shown in FIG. 1;

FIG. 3 is a partly cross sectional side elevational view showing a column base structure 70 according to a second embodiment of the present invention;

FIG. 4 is a partly cross sectional side elevational view showing conventional column base structures 2, 2A and 2B;

FIG. 5 is a conceptual side elevational view for explaining a state in which a load generating a bending moment M is applied in the conventional column base structure 2A shown in FIG. 4; and

FIG. 6 is a conceptual side elevational view for explaining a state in which the load generating the bending moment M is applied in the conventional column base structure 2B shown in FIG. 4.

DESCRIPTION OF REFERENCE NUMERALS

-   2, 2A, 2B column base structure -   3 base concrete -   4 steel frame column -   5 column base portion -   6 joint metal fitting -   6 a upper surface -   6 b bolt insertion hole -   8 mortar -   10 anchor bolt -   12, 14 nut member -   16 washer -   18 fixing plate -   40 column base structure -   41 column base portion -   42 joint metal fitting -   42 a upper surface -   42 b bolt insertion hole -   70 column base structure -   71 column base portion -   72 joint metal fitting -   72 a bottom plate portion -   72 b upper surface -   72 c bolt insertion hole -   72 d support table portion -   72 e upper surface -   F1 design basis strength of joint metal fitting 6 -   F1′ design basis strength of joint metal fitting 42 -   F2 design basis strength of steel frame column 4 -   M bending moment -   Mp bending bearing force of steel frame column 4 -   Mp′ index bending bearing force of steel frame column 4 -   Mu bending bearing force of column base portion 5 -   Mu′ bending bearing force of column base portion 41 -   Np axial bearing force of steel frame column 4 -   Np′ index axial bearing force of steel frame column 4 -   Nu axial bearing force of column base portion 5 -   Nu′ axial bearing force of column base portion 41 -   Ms design final bending bearing force -   W weld portion

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A description will be specifically given below of a mode for carrying out a column base structure according to the present invention with reference to the accompanying drawings.

FIGS. 1 and 2 are views which are referred for explaining a column base structure 40 according to a first embodiment of the present invention. Hereinafter, a description will be given by attaching the same reference numerals to the same portions as those in the conventional column base structure 2, and an overlapping description of the same structures as the conventional ones will be omitted except a part of them.

The column base structure 40 according to the present embodiment is structured such as to be provided with a column base portion 41 which is provided on a base concrete 3, and a steel frame column 4 (a column member) which is joined its lower end portion to a joint metal fitting 42 of the column base portion 41, as shown in FIG. 1. The column base portion 41 has a joint metal fitting 42, a mortar 8, anchor bolts 10, nut members 12 and 14, washers 16 and fixing plates 18.

In the column base portion 41 of the column base structure 40 according to the present embodiment, the tabular joint metal fitting 42 is provided above the base structure 3 via the mortar 8. The joint metal fitting 42 is made of a metal and has both front and back square surfaces.

In the joint metal fitting 42 of the column base portion 41, a lower end portion of the steel frame column 4 is brought into contact with an upper surface 42 a thereof, and the joint metal fitting 42 and the steel frame column 4 are joined to each other by welding in weld portions W. The steel frame column 4 has a length in a vertical direction in FIG. 1 and is formed into a hollow rectangular tubular shape.

Further, an upper end portion of the anchor bolt 10 protruding upward from an inner side of the base concrete 3 is inserted into a bolt insertion hole 42 b which is formed in a peripheral edge portion of the joint metal fitting 42.

A male thread portion which protrudes upward in relation to the joint metal fitting 42 and is formed in the upper end portion of the anchor bolt 10 is inserted to a through hole of the washer 16 and is threadably fastened to a female thread portion of the nut member 12, whereby the steel frame column 4 is provided in a rising manner on the base concrete 3 via the weld portion W, the joint metal fitting 42 of the column base portion 41 and the mortar 8 so as to be fixed.

In the column base structure 40 according to the present embodiment, a design basis strength F1′ of the joint metal fitting 42 is smaller than a design basis strength F2 of the steel frame column 4.

Further, in the column base structure 40 according to the present embodiment, a bending bearing force Mu′ of the column base portion 41 is smaller than an index bending bearing force Mp′ of the steel frame column 4.

Here, the bending bearing force Mu′ of the column base portion 41 is defined by a strength of the joint metal fitting 42, a strength of the anchor bolt 10, and a fixing performance of the joint metal fitting 42 to the base concrete 3 via the mortar 8, the anchor bolt 10, the nut members 12 and 14, the washer 16 and the fixing plate 18, in the same manner as the bending bearing force Mu of the column base portion 5 in the conventional column base structure 2.

On the contrary, the index bending bearing force Mp′ of the steel frame column 4 is calculated by multiplying the bending bearing force Mp of the steel frame column 4 by a value obtained by dividing the design basis strength F1′ of the joint metal fitting 42 by the design basis strength F2 of the steel frame column 4.

The column base structure 40 according to the present embodiment mentioned above is structured such that the joint metal fitting 42 yields and plastically deforms, without local yielding of the weld portion W joining the steel frame column 4 and the joint metal fitting 42 and breakage of the weld portion W, in the case that a load generating a great bending moment M is applied to the steel frame column 4 in a clockwise direction around a center O of rotation of the center portion between a pair of weld portions W to the joint metal fitting 42, as shown in FIG. 2, by making the bending bearing force Mu′ of the column base portion 41 smaller than the index bending bearing force Mp′ of the steel frame column 4.

Further, the building construction provided with the column base structure 40 according to the present embodiment is structured such that a structural calculation is carried out by a structural calculation device employing a structural calculation program.

In other words, the structural calculation device which is used in the building construction provided with the column base structure 40 according to the present embodiment calculates values of the bending bearing force Mp and the axial bearing force Np of the steel frame column 4, the index bending bearing force Mp′ and the index axial bearing force Np′ of the steel column 4, and the bending bearing force Mu′ and the axial bearing force Nu′ of the column base portion 41 according to the structural calculation program, on the basis of the input data. The index bending bearing force Mp′ and the index axial bearing force Np′ are calculated by multiplying values of the bending bearing force Mp and the axial bearing force Np of the steel frame column 4 by a value obtained by dividing the design basis strength F1′ of the joint metal fitting 42 by the design basis strength F2 of the steel frame column 4.

Further, the structural calculation device is structured such that the structural calculation of the building construction is carried out by setting the smaller one of the index bending bearing force Mp′ of the steel frame column 4 and the bending bearing force Mu′ of the column base portion 41 as the final bending bearing force Ms of the column base structure 40, and employing the final bending bearing force Ms as the reference of the admission decision, according to the structural calculation program.

According to the structural calculation device using the structural calculation program mentioned above, the structural calculation is carried out by employing the bending bearing force Mu′ of the column base portion 41 which is smaller than the index bending bearing force Mp′ of the steel frame column 4 as the final bending bearing force Ms, and the dimension of each of the members of the building construction provided with the column base structure 40 is decided.

Therefore, the column base structure 40 according to the present embodiment can prevent the weld portion W joining the steel frame column 4 and the joint metal fitting 42 from locally yielding and prevent the weld portion W from being broken, in the case that the load generating the bending moment M is applied due to the earthquake shown in FIG. 2, even if the design basis strength F1′ of the joint metal fitting 42 of the column base structure 40 is smaller than the design basis strength F2 of the steel frame column 4.

Further, since the design basis strength F1′ of the joint metal fitting 42 can be made smaller than the design basis strength F2 of the steel frame column 4, it is not necessary to employ a high strength material having a great value of the design basis strength F1′ as a material of the joint metal fitting 42, even in the case that the high strength material having the great value of the design basis strength F2 is employed as the material of the steel frame column 4.

As a result, the column base structure 40 according to the present embodiment can prevent the manufacturing cost of the joint metal fitting 42 from becoming higher, and can prevent the manufacturing cost of the column base structure 40 using the joint metal fitting 42 from becoming higher.

As described above, on the basis of the column base structure 40 according to the first embodiment of the present invention, it is possible to prevent the joint portion (the weld portion W) between the joint metal fitting 42 and the steel frame column 4 from locally yielding and prevent the joint portion from being broken, in the case that the load generating the bending moment M is applied due to the earthquake, and it is possible to prevent the manufacturing cost of the joint metal fitting 42 and the column base structure 40 from becoming higher.

FIG. 3 is a view which is referred for explaining a column base structure 70 according to a second embodiment of the present invention.

The column base structure 70 according to the present embodiment is different from the column base structure 40 according to the first embodiment in a point that the column base structure 70 is provided with a joint metal fitting 72 having a bottom plate portion 72 a and a support table portion 72 d as shown in FIG. 3, in place of the joint metal fitting 42 in the column base structure 40 according to the first embodiment.

In other words, the joint metal fitting 72 of the column base portion 71 in the present embodiment is made of a metal, and is constructed by the bottom plate portion 72 a which has both front and back surfaces formed approximately into a square shape and is formed into a plate shape, and the support table portion 72 d which is structured such that a center portion of an upper surface 72 b of the bottom plate portion 72 a has a height toward an upward direction in the drawing in relation to a peripheral edge portion thereof, as shown in FIG. 3.

The joint metal fitting 72 is joined to the steel frame column 4 in the weld portions W by welding, in such a manner that a lower end portion of the steel frame column 4 is brought into contact with an upper surface 72 e of the support table portion 72 d.

Further, an upper end portion of the anchor bolt 10 protruding upward from the inner side of the base concrete 3 is inserted to a bolt insertion hole 72 c which is formed in the peripheral edge portion of the bottom plate portion 72 a of the joint metal fitting 72.

A male thread portion which protrudes upward in relation to peripheral edge portion of the bottom plate portion 72 a in the joint metal fitting 72 and is formed in the upper end portion of the anchor bolt 10 is inserted to a through hole of the washer 16 and is threadably fastened to a female thread portion of the nut member 12, whereby the steel frame column 4 is provided in a rising manner on the base concrete 3 via the weld portion W, the joint metal fitting 72 of the column base portion 71 and the mortar 8 so as to be fixed.

In the column base structure 70 according to the present embodiment, a design basis strength of the joint metal fitting 72 is smaller than the design basis strength F2 of the steel frame column 4, in the same manner as the column base structure 40 according to the first embodiment.

Further, in the column base structure 70 according to the present embodiment, a bending bearing force Mu′ of the column base portion 71 of the column base structure 70 is smaller than the index bending bearing force Mp′ of the steel frame column 4.

On the basis of the column base structure 70 according to the present embodiment mentioned above, it is possible to prevent the joint portion (the weld portion W) between the joint metal fitting 72 and the steel frame column 4 from locally yielding and prevent the joint portion from being broken, in the case that the load generating the bending moment M is applied due to the earthquake, in the same manner as the column base structure 40 according to the first embodiment. Further, since it is not necessary to employ a high strength material having a great design basis strength for the joint metal fitting 72, it is possible to prevent the manufacturing cost of the joint metal fitting 72 and the column base structure 70 provided with the same from becoming higher.

The present invention is not limited only to the embodiments mentioned above, but can be variously modified about the column base structure within a range that can achieve the object of the present invention.

For example, in the column base structure 40 according to the first embodiment mentioned above, the description is given of the case that the joint metal fitting 42 has both the front and back surfaces formed approximately into the square shape and is formed into the tabular shape, however, both the front and back surfaces may be formed into the other rectangular shapes than the square and have different lengths vertically and transversely.

Further, the joint metal fitting 42 may be formed into a tabular shape having both front and back surfaces which are formed into the other polygonal shapes than the rectangular shapes or a circular shape. In this regard, same applies to the bottom plate portion 72 a of the joint metal fitting 72 in the second embodiment mentioned above.

Further, in the column base structure 40 according to the first embodiment mentioned above, the steel frame column 4 which is joined its lower end portion to the joint metal fitting 42 is formed into the rectangular tubular shape, however, is not limited to this shape, but may be formed, for example, as a cylindrical shape. Further, the steel frame column may be formed into a solid shape.

Further, in the building construction provided with the column base structure 40 according to the first embodiment mentioned above, the dimension of each of the members is decided on the basis of the structural calculation carried out by the structural calculation device using the structural calculation program, however, it is not necessary to use the structural calculation program or the structural calculation device, as long as the design basis strength Ft of the joint metal fitting 42 is smaller than the design basis strength F2 of the steel frame column 4, and the bending bearing force Mu′ of the column base portion 41 is smaller than the index bending bearing force Mp′ of the steel frame column 4. 

What is claimed is:
 1. A column base structure comprising: a column base portion in which a joint metal fitting arranged above a base concrete is fixed to an anchor bolt protruding upward from said base concrete; and a column member which is joined its lower end portion to an upper surface of said joint metal fitting, wherein a design basis strength of said joint metal fitting is smaller than a design basis strength of said column member, wherein a bending bearing force of said column base portion is smaller than an index bending bearing force of said column member which is calculated by multiplying a bending bearing force of said column member by a value obtained by dividing the design basis strength of said joint metal fitting by the design basis strength of said column member, and wherein said column base portion yields before said column member according to an increase of the load applied to said column base structure.
 2. The column base structure according to claim 1, wherein said joint metal fitting is formed into a plate shape which has both front and back surfaces and a uniform thickness.
 3. The column base structure according to claim 1, wherein said joint metal fitting has a bottom plate portion which has both front and back surfaces and a uniform thickness, and is formed into a plate shape, and a support table portion which has a height toward an upward direction in an inner side of a peripheral edge portion of an upper surface of the bottom plate portion, and a lower end portion of said column member is joined to an upper surface of the support table portion. 